PPH - what help have you got regarding the breastfeeding. Have you spoken to a counsellor? You haven't said in your post whether you have or not. I personally have helped friends through this situation. Jasper was one who had a baby constantly losing weight and did not regain his birthweight until 3 weeks. The situation was turned around and domperidone helped.
Action plan is needed, preferably with a counsellor who can help your confidence. In the absence of that however, some things you can try.
Feed 2-3 hourly if possible. Ensure he spends his time sucking effectively at the breast. Make sure he is latched on properly throughout.
If you really feel that your breasts are not filling get some Domperidone (Motilium) from the chemist. You can buy it over the counter. Take one 10mg tablet, 3 times a day for 10 days. Our hospital prescribes it and so do the GPs who are awre of it's use. It is actually for bloatedness but a side effect is increased milk production.
Is he passing urine? Is he responsive to you? Are his nappies changing colour? If so, he is getting milk.
With increased feeds you should start to feel a difference in 24-36 hours.
Who is weighing the baby? Is it the midwife or health visitor? Have the last 2 weights been on the same scales? Was he naked? Are you getting good advice from a professional?
All is not lost. This can be turned aroung but you definately need the support from someone who is very experienced in B/F problems.
If it takes the pressure off you, by all means give him some formula. It will fill him up and therefore he won't go to the breast as frequently so express instead. The point of the expressing is stimulation mainly. Do you know how to hand express? Do you have a hand pump?
Sorry, lots of questions. There are lots of things that need to be known in order to help. Please call a counsellor who can help directly.